Polypropylene（PP) Injection Molding Process Details.

PP is commonly known as polypropylene. It is also called "100% glue" because of its good break resistance. PP is a translucent, semi-crystalline thermoplastic with the characteristics of high strength, good insulation, low water absorption, high heat distortion temperature, low density, and high crystallinity. Modified fillers usually include glass fiber, mineral fillers, thermoplastic rubber, etc.

The fluidity of PP for different uses is quite different. The flow rate of PP generally used is between that of ABS and PC.

Pure PP is translucent ivory white and can be dyed into various colors. For PP dyeing, only color masterbatch can be used on general injection molding machines. Some machines have independent plasticizing elements that enhance the mixing effect, and can also be dyed with toner. Products for outdoor use generally use UV stabilizers and carbon black filling. The proportion of recycled materials should not exceed 15%, otherwise it will cause strength reduction and decomposition and discoloration. PP injection molding generally does not require special drying treatment before processing.

There are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. A computerized injection molding machine with higher injection pressure and multi-stage control is required. The clamping force is generally determined as 3800t/m2, and the injection volume is 20%-85%.

The mold temperature is 50-90°C. For those with higher size requirements, use a higher mold temperature. The core temperature is more than 5℃ lower than the cavity temperature, the runner diameter is 4-7mm, the needle gate length is 1-1.5mm, and the diameter can be as small as 0.7mm. The shorter the length of the edge gate, the better, which is about 0.7mm. The depth is half of the wall thickness and the width is twice the wall thickness. It increases gradually with the length of the melt flow in the mold cavity. The mold must have good venting properties. The vent hole must be 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, a large and round sprue and circular flow channel should be used, and the thickness of the reinforcing rib should be small ( For example, 50-60% of the wall thickness). The thickness of products made of homopolymer PP cannot exceed 3mm, otherwise there will be bubbles (thick-walled products can only use copolymer PP).

The melting point of P is 160-175°C, and the decomposition temperature is 350°C. However, the temperature setting cannot exceed 275°C during injection processing, and the melting section temperature is preferably 240°C.

In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and molds are not suitable (bubbles and air marks appear). If the patterned surface appears with alternating light and dark stripes spread from the gate, low injection speed and higher mold temperature should be used.

5bar melt back pressure can be used, and the back pressure of toner can be adjusted appropriately.

Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of the injection pressure). Switch to holding pressure at about 95% of the full stroke and use a longer holding time.

In order to prevent shrinkage deformation caused by post-crystallization, products generally need to be soaked in hot water.